Valenta pulled off a quintuple-twisting triple flip on his last jump to win gold for the Czech Republic.
It was a daring move by the 29-year-old who had been lying in fifth after the first round and earned him a combined score of 257.02 points.
Hometown favourite Joe Pack grabbed silver with 251.64 points, while reigning freestyle skiing world champion Alexie Grichin of Belarus scored 251.19 to take bronze.
"I wasn't really thinking about doing the jump just in
order to win - I wanted to do it because I can do it"
But it was a disappointing day for defending champion Eric Bergoust of the United States, finishing 12th and last after a poor second jump.
Valenta, who had finished eighth in the qualifying session, seemed intent on leaving his best until last.
He soared high into the air on his final jump, performing five twists and three flips before landing and balanced briefly on one ski and then skiing cleanly down the landing slope.
It not only guaranteed him gold, but also a place in the history books.
"Maybe I changed the sport a little bit," he said afterwards.
"The guys are going to say, 'If Ales can do it, we can do it too.'"
"I wasn't really thinking about doing the jump just in order to win. I wanted to do it because I can do it."